Angular *ngFor，有条件显示“类别”下的子类别

我试图在angular 2应用程序中显示带有JSON响应的类别下的子类别。我将ngIf用于过滤器子类别。如果parent_id不为null，则其子类别以JSON字符串表示

这是我的JSON字符串

[
  {
    "id": "15",
    "parent_id": null,
    "category": "Dress"
  },
  {
    "id": "17",
    "parent_id": "15",
    "category": "Dress"
  },


  {
    "id": "19",
    "parent_id": null,
    "category": "Men"
  },
  {
    "id": "30",
    "parent_id": "19",
    "category": "none"
  },
  {
    "id": "31",
    "parent_id": "19",
    "category": "Shorts"
  }
]

模板

<ul class="navbar-nav">
        <span *ngFor="let category of categories">
      <li class="nav-item dropdown">
      <a class="nav-link dropdown-toggle" href="http://example.com" id="navbarDropdownMenuLink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false" >
      {{category.category}}
      </a>
      <div class="dropdown-menu" aria-labelledby="navbarDropdownMenuLink">
        <span *ngFor="let subcategory of categories" *ngIf="subcategory.parent_id==category.id">
      <a class="dropdown-item" href="#">
       {{subcategory.category}}     
        </a>
         </span>
      </div>
      </li>
        </span>
      </ul>

非常感谢您的帮助……

    <ul class="menu_inline">
        <li class="menu">
            <div *ngFor="let category of category_list">
            <a>{{ category.name }}</a>
            <ul>
                <li><div *ngFor="let subcategory of subcategory_list" (click)="onSelects(subcategory)">
                <ng-container *ngIf="subcategory.category==category.id">
                <a>{{subcategory.name}}</a>
                </ng-container></div>
                </li>
            </ul>
            </div>
        </li>
    </ul>


{{category.name}


{{subcategory.name}

我无法使用上述代码获取类别下的子类别，据我所知，您不能在同一元素中使用

*ngFor

和

*ngIf

。在

*ngFor

中的内部元素中使用

*ngIf

。developer033是绝对正确的，我想我会就此抛出一个文档：）谢谢。。！知道了。。它起作用了